The invention relates to a method and apparatus for the operation of rotary anode x-ray tubes. A method of this type and an apparatus for its realization are e.g. described in the German OS No. 2,455,974.
In the case of rotary anode x-ray tubes, the anode, together with the rotor connected with it via the drive shaft, is supplied with a high voltage potential. The stator, resting externally against the tube, is normally at ground potential, since otherwise, as in the case of an embodiment according to U.S. Pat. No. 4,107,535, the drive voltage would have to be supplied via a high voltage transformer and high voltage cable. The problem is only that a high voltage insulation must be carried out between the rotor of the anode and the stator. This is particularly problematical, on account of the large air gap brought about thereby, when high operating voltages (e.g. 150 kV) are to be connected in unipolar fashion to cathode-side grounded tubes, such as e.g. expedient in the case of grid-controlled tubes on account of the simple controllability of the cathodes.
A known solution is disclosed in the above cited German OS No. 2,455,974. It consists in that the rotor of the anode assembly is insulated relative to the anode itself within the tube, and that said rotor in operation carries approximately the same potential as the stator, preferably ground potential. However, this only shifts the insulation problems into the tube interior and therefore requires complicated and expensive tube constructions.
During operation of a rotary anode x-ray tube, the fluctuating magnetic field of the rotary anode induction motor causes a disturbance of the electron beam which results in a migration back and forth of the focal spot on the anode in synchronism with the magnetic field fluctuations. In order to avoid these fluctuations, which bring about an undesired effective expansion of the focal spot, accordingly, in accordance with the German LP No. 929,142, for a diagnostic x-ray apparatus with a rotary anode, switching means coupled with the switching-on means for switching-on the x-ray tube, have been proposed, which switching means, immediately prior to the radiograph, respectively, effect a disconnection of the current supply to the stator of the rotary anode motor previously brought to the prescribed rotational rate.